Catheter calibration and usage monitoring system

ABSTRACT

A probe ( 20 ) for insertion into the body of a subject, the probe ( 20 ) having distal ( 22 ) and proximal ends, and including an electronic microcircuit ( 90 ), which stores information relating to calibration of the probe ( 20 ). Preferably, the microcircuit ( 90 ) stores a calibration code, which is encrypted. Alternatively or additionally, the microcircuit ( 90 ) stores a usage code, which controls availability of the probe ( 20 ) to a user thereof. Preferably, the probe ( 20 ) includes access control circuitry ( 90 ) that allows the usage code to be changed so as to reduce the availability of the probe ( 20 ), but not to increase the availability thereof.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional patentapplications No. 60/011,723, titled “Catheter Calibration System”, filedFeb. 15, 1996 and No. 60/017,635, titled “Catheter Calibration System”,filed May 17, 1996, the disclosures of both of which incorporated hereinby reference. This application is also related to a PCT applicationtitled “Medical Procedures and Apparatus Using Intrabody probes”, filedon even date by applicant Biosense Inc., in the U.S. receiving officeand designating, inter alia, the U.S.

FIELD OF THE INVENTION

The present invention relates generally to systems for medical diagnosisand treatment, and specifically to medical catheters whose location canbe detected.

BACKGROUND OF THE INVENTION

Various methods and devices have been described for determining theposition of a probe or catheter tip inside the body usingelectromagnetic fields, such as in U.S. Pat. No. 5,042,486 and PCTpatent publication No. WO 94/0938, whose disclosures are incorporatedherein by reference. Other electromagnetic tracking systems, notnecessarily for medical applications, are described in U.S. Pat. Nos.3,644,825, 3,868,565, 4,017,858, 4,054,881 and 4,849,692, whosedisclosures are likewise incorporated herein by reference.

U.S. Pat. No. 5,391,199, filed Jul. 20, 1993, which is assigned to theassignee of the present patent application and whose disclosure isincorporated herein by reference, describes a system that incorporates acatheter, which includes a position measuring device that can determinethe position of the catheter in three dimensions, but not itsorientation.

PCT patent application No. PCT/US95/01103, which is likewise assigned tothe assignee of the present patent application and whose disclosure isincorporated herein by reference, describes a catheter system includingmeans for determining the six-dimensions of position and orientation ofthe catheter's distal tip. This system uses a plurality ofnon-concentric coils adjacent to a locatable site in the catheter, forexample near its distal tip. Preferably three orthogonal coils are used.These coils generate signals in response to externally applied magneticfields, which allow for the computation of six position and orientationcoordinates, so that the position and orientation of the catheter areknown without the need for imaging the catheter.

U.S. Pat. No. 5,383,874 (Jackson et al.) describes a system foridentifying and monitoring catheters, including identification meanscarried within the handle of the catheter body. In one embodiment of theinvention of this patent, the handle includes a solid-state microchippre-programmed with a digital value representing the catheter'sidentification code and other operational and functional characteristicsof the catheter. The handle is connected by a cable to a controlconsole, which reads data from the microchip. In one disclosedembodiment, the microchip may record the number of times the catheterhas been used. Digital data storage in the catheter handle adds multipledigital signal wires to the catheter.

SUMMARY OF THE INVENTION

The coils of the '103 patent application and other systems forelectromagnetic detection of catheter position and orientation aregenerally located in the catheter at a small distance proximal to thecatheter's distal tip, since the distal tip is typically occupied by anelectrode or other functional element. Therefore, the position andorientation detection system must be calibrated to take into account thedisplacement of the distal tip of the catheter relative to the locationof the coils. Because of manufacturing variations, this displacementgenerally varies from one catheter to another.

Furthermore, the coils used to generate position signals may not beprecisely orthogonal. For purposes of computing the position andorientation of the catheter, the axes of the coils define the respectiveaxes of a coordinate system that is fixed to the catheter tip, and thedirections of these axes must be known relative to the catheter. Ifthese axes deviate from orthogonality, the respective degrees ofdeviation must be known and corrected for in the position andorientation computation.

Additionally, the relative gains of the coils determine the strengths ofthe respective position signals that the coils generate in response toexternally-applied fields. Since these signal strengths are used incomputing the position and orientation of the catheter, deviations ofthe gains from their expected values will lead to inaccuracy in thecomputed position and orientation. Therefore, the respective gains ofthe coils must be known and corrected for in the position andorientation computation.

It would, therefore, be desirable to pre-calibrate the catheter,preferably at the time of manufacture, so as to measure and compensatefor variations in the positions, orientations and gains of coils used togenerate position signals.

Preferably the calibration data should be recorded in such a way as toalleviate the need for recalibration and manual entry of calibrationdata before each use.

It is, therefore, an object of the present invention to provide a methodof calibrating a device that is used to determine the position andorientation of a catheter, wherein the calibration information isretained in the catheter.

A further object of the present invention is to provide means forconvenient electronic storage and recall of calibration informationregarding a catheter.

In one aspect of the present invention, this calibration information isstored digitally in a microcircuit whose location is easily accessibleto signal processing circuits and computing apparatus, so that thecatheter need not contain digital signal wires, and digital electronicsignals transmitted from the microcircuit to the signal processingcircuits and computing apparatus do not interfere with low-level analogsignals conveyed by wires from the distal end of the catheter to thecircuits.

In preferred embodiments of the present invention, a device used todetermine the position and orientation of a catheter inside the bodycomprises a plurality of coils adjacent to the distal end of thecatheter. The catheter further comprises an electronic microcircuitadjacent to the proximal end of the catheter, which microcircuit storesinformation relating to the calibration of the device.

Preferably the microcircuit comprises a read/write memory component,such as an EEPROM, EPROM, PROM, Flash ROM or non-volatile RAM, and theinformation is stored in digital form.

In preferred embodiments of the present invention, this calibrationinformation includes data relating to the relative displacement of thedistal tip of the catheter from the coils. In some other preferredembodiments of the present invention, the calibration information alsoincludes data relating to deviation of the coils from orthogonality, ordata relating to the respective gains of the coils, or a combination ofthese data.

In some preferred embodiments of the present invention, in which thecatheter is electrically isolated from signal processing and computingapparatus, the calibration information includes data relating toisolation circuitry in the catheter. Preferably, the catheter isisolated by at least one inductive element, such as an isolationtransformer, adjacent to the proximal end of the catheter or in a handleassociated with the catheter. Alternatively, the catheter may beisolated by one or more opto-isolaters, or other types of isolationcircuitry known in the art. Such inductive elements and other isolationcircuitry typically introduce non-linearities in signals conveyedthereby. Such non-linearities may lead to significant distortionsparticularly in analog signals conveyed by wires from the distal end ofthe catheter to the signal processing circuits. Therefore, thecalibration information preferably includes data relating to signalnon-linearities introduced by the inductive elements and/or otherisolation circuitry.

In a preferred embodiment of the invention, the catheter is a wirelesscatheter which is not physically connected to the signal processingand/or computing apparatus. Rather, a transmitter/receiver is attachedto a proximal end of the catheter. The transmitter/receiver communicateswith the signal processing and/or computer apparatus using wirelesscommunication methods, such as IR (infra red), RF or acoustictransmissions. One benefit of this type of configuration is that thecatheter, which is inserted into the (electrically sensitive) heart caneasily be made electrically floating. Another benefit is a reduction inthe amount of cabling and wiring which in which one of the manyoperators might get entangled and/or accidentally pull out of the body.Still another advantage is the ease of sterilizing and maintaining thesterility of such a catheter, since the entire catheter may besterilized as a single unit. In a preferred embodiment of the invention,the proximal end of the catheter, which includes thetransmitter/receiver, is attached to an operator's belt. Preferably,there is a handle disposed on the catheter, a few feet away from theproximal end thereof, for control of the catheter. As can beappreciated, when such a catheter is used for ablation or for infusionof materials into the body, it is preferably momentarily connected to anexternal device, such as an RF generator.

In preferred embodiments of the present invention, the microcircuit iscontained in a connector at the proximal end of the catheter. Preferablythis connector couples electronic signals from the catheter to signalprocessing circuits and computing apparatus.

In preferred embodiments of the present invention, electronic signalscoupled by the connector comprise both digital and analog signals.Furthermore, in some preferred embodiments of the present invention, theanalog signals include both electrophysiological signals received byelectrodes in the catheter and position and orientation signalsgenerated by the coils. Preferably the position and orientation signalsare conveyed by twisted wire pairs or shielded wires, and the connectoralso includes shielding to reduce noise and interference in thesesignals.

In other preferred embodiments of the present invention, the catheterincludes one or more analog-to-digital (A/D) converter circuits, whichconvert electrophysiological signals and position and orientationsignals from analog to digital form. In these embodiments, the connectorcouples only digital signals from the catheter to the signal processingcircuits and computing apparatus. In one such preferred embodiment, anA/D converter is adjacent to the distal tip of the catheter. In othersuch preferred embodiments, an A/D converter is adjacent to the proximalend of the catheter, for example, in a handle attached to the catheteror in the connector.

Preferred embodiments of the present invention further provide a methodof calibrating a device used to detect the position and orientation ofthe distal tip of a catheter, wherein the calibration information isstored in the catheter. Prior to operation of the device, a computerreads the stored calibration information and uses this information indetermining the position and orientation of the catheter inside thebody.

In preferred embodiments of the present invention in which the deviceused to determine position and orientation comprises coils adjacent tothe distal end of the catheter, calibration information regarding therespective gains and orientations of the coils is generated by placingthe distal end of the catheter in a known, predetermined position andorientation and applying to it known magnetic fields. The coils generatesignals in response to the magnetic fields, which signals are detectedand compared to normal signal values in order to calculate calibrationdata. These calibration data are then used to correct subsequentposition and orientation determinations, so as to account for thedeviation of the gains and orientations of the coils from normal values.

Furthermore, in preferred embodiments of the present invention,calibration information regarding the displacement of the distal tip ofthe catheter relative to the coils is generated by placing the distaltip of the catheter in one or more predetermined positions andorientations and applying known magnetic fields to the catheter. Thesignals generated by the coils in response to the magnetic fields aredetected and used to calculate a correction function, which may be usedsubsequently in the determination of the position and orientation of thedistal tip of the catheter.

In preferred embodiments of the present invention, a mechanical jigholds the catheter in one or more predetermined positions andorientations during calibration, and radiators generate known,substantially uniform magnetic fields in the vicinity of this jig.Signals generated by the coils are analyzed and used to producecalibration data regarding the gains of the coils and deviations of thecoils from orthogonality.

In other preferred embodiments of the present invention, a mechanicaljig holds the catheter in a plurality of predetermined positions andorientations during calibration. Radiators generate predetermined,non-uniform magnetic fields in the vicinity of this jig, wherein themagnetic field strengths and directions are known as functions ofposition in the jig. Signals generated by the coils are analyzed andused to produce calibration data regarding the respective displacementsof the coils relative to the tip of the catheter.

In some preferred embodiments of the present invention, apparatus foruse in calibrating the device for detecting the catheter's position andorientation includes a heater and temperature sensor, which maintain thecatheter's distal tip at a predetermined, known temperature duringcalibration. Preferably, the tip is maintained at the temperature of thebody into which the catheter is to be inserted, for example 37° C. Inthis way, temperature-related errors in calibration, for example, due totemperature-related changes in the inductance of the coils in thecatheter, may be avoided.

The calibration data that are produced in accordance with any of theabove preferred embodiments may be recorded in the form of lookuptables, polynomial coefficients or other forms known in the art, whichare then stored in a microcircuit in the catheter.

In preferred embodiments of the present invention, calibration data areproduced and recorded at the time of manufacture, and the microcircuitis configured so as to prevent subsequent recording of calibration databy a user. In some such preferred embodiments of the present invention,the microcircuit comprises an EPROM or PROM device, which is containedin a connector at the proximal end of a catheter, and the input andoutput connections of the EPROM or PROM are coupled to pins of theconnector. Calibration data are recorded in the EPROM or PROM at thetime of manufacture using a suitable programming device, which receivesdata from a computer used in calibration. The EPROM or PROM programmingdevice is connected to the catheter connector and programs the EPROM orPROM by inputting digital signals thereto through the connector.Thereafter, the EPROM or PROM may not be re-programmed.

In other such preferred embodiments of the present invention, whereinthe microcircuit comprises an EEPROM or non-volatile RAM device, theEEPROM or non-volatile RAM device includes a write-enable inputconnection, of a type known in the art, which is connected to awrite-enable pin in a connector at the proximal end of a catheter. Atthe time of calibration, the write-enable input is enabled, andcalibration data are recorded in the microcircuit. Thereafter thewrite-enable input is disabled, for example by removing the write-enablepin or by connecting it to electrical ground, so that furthercalibration data may not be recorded in the microcircuit.

Alternatively, in preferred embodiments of the present invention whereinthe microcircuit comprises an EEPROM device, the write-enable input maybe disabled by sending a write-protect command to the device. Thiscommand may be reversible or irreversible.

In still other preferred embodiments of the present invention, themicrocircuit comprises access control circuitry, such as, for example,the X76F041 Password Access Security Supervisor (PASS™) SecureFlash ROMdevice, manufactured by Xicor, Inc. The microcircuit is preferablyprogrammed with a password, so that after calibration data are producedand recorded at the time of manufacture, further calibration data maynot be recorded in the microcircuit, with the possible exception of datarecording by factory-authorized personnel to whom the password is known.

In some preferred embodiments of the present invention, data recorded inthe microcircuit include a calibration code, which is encrypted inaccordance with methods known in the art, so as to ensure thatcalibration data have not been altered or corrupted. When a userconnects the catheter to a suitable console, which console comprises acomputer, the computer reads the calibration code and compares the codewith pre-programmed values. If the code does not match the desiredpre-programmed value, the computer causes a message to be displayedindicating that the catheter may not be appropriately calibrated. Thecomputer may prevent further operation until a catheter having a codematching the desired pre-programmed value is connected thereto.

Preferably the calibration code is encrypted using a method thatprevents decryption by unauthorized parties, for example the RSAencryption scheme, using a public key and a private key, or othermethods known in the art. When a method such as RSA encryption is used,the private key is known only to authorized manufacturers of thecatheter, so as to prevent the possible use of unauthorized substitutesof possibly inferior quality.

In further preferred embodiments of the present invention, data recordedin the microcircuit include an expiration date and time, after which thecatheter may not be used. When a user connects the catheter to asuitable console, which console comprises a computer, the computer readsthe expiration date and time and compares then to the actual date andtime, generated, for example, by a real-time clock circuit. If theexpiration date and time have passed, the computer causes a message tobe displayed indicating that the catheter is unsuitable for further use.The computer may prevent further operation until a catheter having avalid expiration date and time is connected thereto.

Preferably the expiration date and time are recorded by the consolecomputer by programming the microcircuit in the catheter when thecatheter is first used. Thus, when the catheter is connected to aconsole for the first time, the computer detects that no expiration dateand time have yet been recorded in the microcircuit, and programs themicrocircuit with the appropriate expiration data and time, at a pre-setinterval after the actual date and time. The pre-set interval ispreferably determined by the manufacturer, based on the expected usefullife of the catheter.

In a preferred embodiment in which the microcircuit comprises accesscontrol circuitry, the microcircuit is programmed so that a memorylocation therein is operable in a “read access and program only” mode.The mode may be changed only by entry of an appropriate password, whichis generally not available to users of the system. In the “read accessand program only” mode, a number stored in the memory location may bedecreased, by changing a bit from “1” to “0”, but not increased, sincethe microcircuit as programmed will not permit a “0” to be changed to a“1”. Preferably the memory location is set at the time of manufacture tocontain a maximum value, i.e., all bits set to “1”. Then, as describedabove, at the time of first use, the computer programs the microcircuitwith the appropriate expiration time and date by changing one or morebits in the register from “1” to “0”. Thereafter, the expiration datecannot be changed to any later date (unless the correct password isfirst entered).

Alternatively or additionally, the microcircuit comprising accesscontrol circuitry, as described above, may be used to track the numberof times the catheter has been used and/or the duration of use, in amanner that is protected from possible tampering or error by a userthereof. Preferably, a record corresponding to the number of timesand/or the length of time that the catheter may be used is stored in amemory location in the device at the time of manufacture, and themicrocircuit is programmed so that this memory location is operable inthe “read access and program only” mode, as described above. Each timethe catheter is used, and/or at regular time intervals during use, thecomputer reads the record in the memory location and reduces it bychanging one or more bits therein from “1” to “0”. When the recordstored in the memory location reaches zero, or some other predeterminedminimum value, the computer causes a message to be displayed to the userindicating that the catheter is unsuitable for further use and,preferably, prevents further operation until a suitable catheter isconnected thereto.

There is therefore provided in accordance with a preferred embodiment ofthe present invention, a probe for insertion into the body of a subject,the probe having distal and proximal ends, and including an electronicmicrocircuit, which stores information relating to calibration of theprobe. Preferably the microcircuit stores a calibration code, which isencrypted.

Preferably, the microcircuit stores a usage code, which controlsavailability of the probe to a user thereof, and the probe includesaccess control circuitry that allows the usage code to be changed so asto reduce the availability of the probe, but not to increase theavailability thereof. The microcircuit preferably stores the usage codein a memory location therein that is controlled by the access circuitryso as to operate in a read access and program only mode, which mode maybe changed by entry of a password to the access control circuitry.Preferably, the usage code includes date information. Preferably, theprobe includes a device that generates signals responsive to theposition or orientation of the probe, and the information relating tocalibration of the probe includes information relating to calibration ofthe signal generating device. Preferably, this device is adjacent to thedistal end of the probe.

Preferably, the signal generating device includes one or more coils, andthe information relating to calibration includes information relating toa gain of at least one of the one or more coils. Furthermore, theinformation relating to calibration preferably includes informationrelating to an angular orientation of at least one of the one or morecoils, and additionally, information relating to a positionaldisplacement of the signal generating device, relative to the distal endof the probe.

In preferred embodiments of the present invention in which the probeincludes isolation circuitry, the information relating to calibrationpreferably includes information relating to a non-linearity of theisolation circuitry. Preferably, the microcircuit is adjacent to theproximal end of the probe. Moreover, the probe preferably includes aconnector at its proximal end, in which the microcircuit is contained.

Additionally, the microcircuit is preferably a programmable memorydevice, which may comprise an EEPROM, non-volatile RAM, EPROM, Flash ROMor PROM device.

There is further provided in accordance with a preferred embodiment ofthe present invention, apparatus for determining the position of a probein the body of a subject, including a probe as described above; and aconsole, including a computer, which receives position- ororientation-responsive signals from the probe and the informationrelating to calibration of the probe, and uses them to determine theposition of the probe.

Preferably, the microcircuit is adjacent to the proximal end of theprobe. Moreover, the probe preferably further includes a connector atits proximal end, in which the microcircuit is contained in theconnector, and the console further includes a mating receptacle, whichis adapted to be coupled with the probe connector.

Preferably, the microcircuit is a programmable memory device, and theprobe includes one or more connections adapted for programming theprogrammable memory device, which may be an EEPROM, non-volatile RAM,EPROM, Flash ROM or PROM device. Additionally, the mating receptaclepreferably includes means for disabling at least one of the connectionsfor programming the programmable memory device.

Preferably, the computer is further adapted to program the programmablememory device. In preferred embodiments of the present invention inwhich the memory device is an EPROM or PROM device, the consolepreferably further includes EPROM or PROM programming apparatus, whichis adapted to program the EPROM or PROM device.

There is further provided in accordance with a preferred embodiment ofthe present invention, a method of calibrating a probe for insertioninto the body of a subject, including determining calibration datarelating to the probe, and programming a microcircuit in the probe so asto record the calibration data in the microcircuit.

Preferably, the method also includes encrypting a calibration code andprogramming the microcircuit with the encrypted code. The methodpreferably further includes reading the encrypted calibration code andnotifying a user of the probe, or ceasing operating of the probe, if theencrypted code does not match a predetermined code.

Preferably, programming the microcircuit includes setting a usagerecord, which is indicative of a first or final use date of the probeand/or of the number of times the probe may be re-used and/or of theremaining duration of time during which the probe may be used.Preferably, when the probe is used the usage record is updated.Preferably, programming the microcircuit includes restricting access tothe usage flag, preferably by setting a password, so that the usagerecord may thereafter be changed so as to reduce availability of theprobe to the user. but not to increase the availability. Preferably, thecalibration data relate to a signal generating device, which generatessignals responsive to the position or orientation of the probe.Preferably, the signal generating device has a gain, and the calibrationdata include data relating to the gain of the device. Alternatively oradditionally, the calibration data may include data relating to anangular orientation of the signal generating device and data relating toa positional displacement of the position- or orientation-responsivesignal generating device, relative to the probe.

There is also provided in accordance with a preferred embodiment of thepresent invention, a method of determining the position or orientationof a probe, including determining calibration data relating to the probeand programming a microcircuit in the probe, in accordance with thepreferred embodiments described above; and computing the position ororientation of the probe inside the body based on the position- ororientation-responsive signals and on the calibration data.

There is also provided in accordance with a preferred embodiment of theinvention, a method of controlling a usage of a probe having anencrypted code stored therein, including reading the encrypted code andnotifying a user of the probe if the encrypted code does not match apredetermined code.

There is further provided in accordance with a preferred embodiment ofthe invention a method of controlling a usage of a probe having anencrypted code stored therein, including reading the encrypted code andceasing operating of the probe if the encrypted code does not match apredetermined code. Alternatively, the code is compared to a range ofvalues. Preferably, the method includes updating the usage record on theprobe.

There is also provided in accordance with a preferred embodiment of theinvention a method of calibrating a probe for insertion into the body ofa subject, including providing a probe having a locatable portion and asignal generating device, which device generates signals responsive tothe position or orientation of the probe, fixedly coupling said signalgenerating device and said locatable portion in one or morepredetermined positions and orientations, applying predeterminedmagnetic fields to the probe, which magnetic fields are known at thevicinity of the signal generating device and which magnetic fields causethe signal generating device to generate the position- ororientation-responsive signals and receiving signals generated by thesignal generating device.

Preferably, at least some of the calibration data are determined byapplying substantially uniform magnetic fields to the probe.Alternatively or additionally, at least some of the calibration data aredetermined by applying spatially variable magnetic fields to the probe.Alternatively or additionally the position- or orientation-responsivesignals generated by the signal generating device have an amplitude,which is characterized by a proportionality to a directional componentof the magnetic fields applied thereto, and the calibration data includedata relating to said proportionality.

Alternatively or additionally, the calibration data include datarelating to an angular orientation of the position- ororientation-responsive signal generating device. Alternatively oradditionally, the calibration data include data relating to a positionaldisplacement of the position- or orientation-responsive signalgenerating device, relative to the probe.

In a preferred embodiment of the invention, the method includes heatingthe probe, preferably, to approximately 37° C.

In a preferred embodiment of the invention, the calibration data isstored on the probe.

There is also provided in accordance with a preferred embodiment of theinvention, apparatus for calibration of a probe having a positionsensing device therein, including a plurality of coils, wherein thecoils define three substantially orthogonal axes and a central region,and are adapted to generate substantially uniform magnetic fields alongthe directions of the three axes in the central region, and means forfixing the distal end of the probe in the central region. Preferably,the coils include three orthogonal pairs of mutually parallel coils.Alternatively or additionally, the apparatus includes a clamp forholding the probe in a fixed position and orientation in the centralregion.

There is further provided in accordance with a preferred embodiment ofthe invention, apparatus for calibration of a probe, having a positionsensing device therein, including a jig, including a plurality ofreceptacles adapted for insertion of the probe thereinto, each saidreceptacles defining a different predetermined position and orientationof the probe and a plurality of coils, wherein the coils generatemagnetic fields that are different for the different predeterminedpositions and orientations.

In a preferred embodiment of the invention, the apparatus includes aheater, which heats the probe. Preferably, the Apparatus includes atemperature sensor, which senses the temperature of the probe.

There is also provided in accordance with a preferred embodiment of theinvention a wireless catheter including an elongate flexible body havinga distal end and a proximal end, a signal generating portion at thedistal end of the body and a transmitter which transmits signalsgenerated by the signal generation portion to an external receiver.Preferably, the transmitter includes a receiver, which receivestransmissions from an external transmitter. Preferably, the aboveapparatus is adapted for calibrating the probe in accordance with themethods described above.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a system including a catheter inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a detailed sectional view of the distal end of the catheter ofFIG. 1;

FIG. 3A is a perspective view of a jig useful in calibrating a catheterin accordance with a preferred embodiment of the present invention;

FIG. 3B is a schematic side view of the jig of FIG. 3A;

FIG. 3C is a perspective view of a catheter clamp for use in conjunctionwith the jig of FIG. 3A;

FIG. 4 is a partially cutaway perspective view of another calibrationjig useful in calibrating a catheter in accordance with a preferredembodiment of the present invention; and

FIG. 5 is a detailed schematic view of a connector at the proximal endof a catheter in accordance with a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a catheter system in accordance with a preferred embodimentof the present invention. The system comprises an elongate probe,preferably a catheter 20, for insertion into the human body. It will beunderstood that although the following preferred embodiments aredescribed with reference to a catheter, the present invention is equallyapplicable to other types of probes.

The distal end 22 of catheter 20 includes a functional portion 24 forperforming diagnostic and/or therapeutic functions, adjacent to distaltip 26. Functional portion 24 may, for example, comprise electrodes (notshown in the figure) for performing electrophysiological measurements orfor electrosurgical ablation of areas of pathology in the heart.Alternatively or additionally, the functional portion may comprise othertypes of sensors, or optical or ultrasound imaging devices.

Distal end 22 of catheter 20 further includes a device 28 that generatessignals used to determine the position and orientation of the catheterwithin the body. Device 28 is preferably adjacent to functional portion24. There is preferably a fixed positional and orientationalrelationship between device 28 and portion 24, at least during thecalibration process.

Catheter 20 preferably includes a handle 30 for operation of thecatheter by a surgeon, wherein controls 32 on handle 30 enable thesurgeon to steer the distal end of the catheter in a desired direction,or to position and/or orient it as desired.

The system shown in FIG. 1 further comprises a console 34, which enablesthe user to observe and regulate the functions of catheter 20. Console34 preferably includes a computer 36, keyboard 38, signal processingcircuits 40, which are typically inside the computer, and display 42.Signal processing circuits 40 typically receive, amplify, filter anddigitize signals from catheter 20, including signals generated byposition signal generating device 28, whereupon these digitized signalsare received and used by computer 36 to compute the position andorientation of the catheter.

Catheter 20 is coupled at its proximal end by connector 44 to a matingreceptacle 46 on console 34. Preferably, catheter 20 further containsone or more isolation transformers (not shown in the figures), whichelectrically isolate the distal portion of the catheter from console 34.The isolation transformers are preferably contained in catheter handle30.

Reference is now made to FIG. 2, which shows a detailed view of distalend 22 of catheter 20 in accordance with a preferred embodiment of thepresent invention. Device 28 comprises three non-concentric coils 60, 62and 64, such as described in PCT patent application No. PCT/US95/01103,now published as WO96/05768, whose disclosure is incorporated herein byreference. This device enables continuous generation of six dimensionsof position and orientation information. Coils 60, 62 and 64 haverespective axes 66, 68 and 70 which preferably define orthogonalCartesian axes Z, X and Y, respectively, as shown in FIG. 2, wherein theZ-axis is parallel to the long axis of catheter 20 and the X- and Y-axesdefine a plane perpendicular thereto. The coils each have a fixedposition and orientation with respect to each other.

Although preferred embodiments of the present invention are describedhere with reference to the position signal generating device shown inFIG. 2 and described above, it will be understood that the inventiveconcepts of the present invention are similarly applicable to probesincluding other position sensing devices. For example, preferredembodiments of the present invention may comprise a single coil forgenerating position signals, or two or more such coils, which may beconcentric or non-concentric. Other preferred embodiments of the presentinvention may comprise other types of position sensing devices, such asHall effect devices.

As shown in FIG. 2, device 28 is located in catheter 20 at a distance Lfrom distal tip 26, where L is here defined for convenience as thedistance along the Z-axis from the central axis 68 of coil 62 to tip 26.Respective axes 66 and 70 of coils 60 and 64 are displaced from axis 68by respective distances dy and d_(z).

When a time-varying external magnetic field is applied to distal end 22of catheter 20, coils 60, 62 and 64 generate analog signals, which arepreferably conveyed through the catheter by coil wires 72. Theamplitudes of these analog signals are typically small relative to otherelectrical signals in and around catheter 20, such as theelectrophysiological signals measured by functional portion 24 andconveyed through the catheter by functional wires 76. Furthermore,external magnetic fields may also cause undesired electrical currents,not generated by coils 60, 62 and 64, to flow in coil wires 72. Theseother electrical signals and undesired electrical currents can causenoise or interfering signals to appear together with the signalsgenerated by the coils. Therefore, in preferred embodiments of thepresent invention, wires 72 are configured as twisted pairs and may alsobe shielded from electromagnetic interference by shields 74, so as tomaintain a high signal-to-noise ratio in the position and orientationsignals received from the coils.

In an alternative preferred embodiment of the present invention, notshown in the figures, catheter 20 further includes one or moreanalog-to-digital (A/D) converters proximate to coils 60, 62 and 64,which convert the analog signals generated by the coils to digital form.In this embodiment, the coil signals are conveyed through the catheterin digital form. Signals measured by functional portion 24 may similarlybe digitized. Thus, fewer wires are necessary to transport the signalsand less of the catheter is taken up by signal wires.

As described in the 01103 PCT patent application, signal processingcircuits 40 in console 34 receive the signals carried by coil wires 72and convey them to computer 36, which computes the three-dimensionaltranslational position of device 28 and the rotational orientation ofaxes 66, 68 and 70, relative to a fixed, external coordinate frame. Theactual position and orientation of distal tip 26 are then computed bytaking into account the distance L of tip 26 from the center of device28, as defined by axis 68, and the orientation of axes 66, 68 and 70.

It has been found empirically that due to deviations in the process ofmanufacturing catheter 20, the distance L typically varies from onecatheter to another, leading to errors in calculating the position oftip 26. Furthermore, axis 66 of coil 60 typically deviates from absolutealignment with the long axis of catheter 20, which passes through tip26, and axes 66 and 70 of coils 60 and 64 respectively are typically notprecisely orthogonal to axis 66 or to each other, thereby inducingadditional errors in determination of position and orientation of thecatheter. Finally, variations in the respective gains of coils 60, 62and 64 and in the distances dy and d_(z) may cause additional errors indetermination of position and orientation of the catheter.

Therefore, in preferred embodiments of the present invention, the device28 that is used to determine the position and orientation of catheter 20is calibrated before the catheter is inserted into a patient's body.Preferably this calibration is performed using one or more jigs, such asthose shown, for example, in FIGS. 3A, 3B and 4.

FIGS. 3A and 3B show a preferred embodiment of a jig 77 for use incalibrating the respective gains and deviations from orthogonality ofcoils 60, 62 and 64. Jig 77 comprises three mutually orthogonal pairs ofparallel radiator coils 79, 81 and 83, mounted on base 85. The radiatorcoils are coupled to radiator driver circuitry, not shown in thefigures, which causes the radiator coils to generate magnetic fields.Each radiator coil pair generates a magnetic field that is substantiallynormal to the planes defined by the pair of coils, and is thussubstantially orthogonal to fields generated by the other two radiatorcoil pairs.

The radiator coils are configured so as to generate predetermined,substantially uniform magnetic fields in a region adjacent to the centerof the jig, i.e., in a region centrally located in between the threepairs of radiator coils. Preferably the driver circuitry is adjusted sothat the amplitudes of the respective magnetic fields generated by threeradiator coil pairs are equal.

As shown in FIG. 3B, jig 77 further comprises a catheter clamp assembly87, which is located inside the jig and not seen in FIG. 3A. As shown inFIG. 3C, clamp assembly 87 includes a clamp base 89, which is fixed toone or more of radiator coils 79, 81 and 83 in a known position andorientation. Preferably clamp assembly 87 is constructed and configuredin jig 77 so that a catheter held in the clamp assembly will be in theregion of substantially uniform magnetic fields adjacent to the centerof the jig, and so that the long axis of the catheter will besubstantially normal to the planes defined by one of the pairs ofparallel radiator coils, for example, coils 83 as shown in FIG. 3B. Aclamp cover 91 is rotatably attached to base 89 by a hinge 93. Base 89and cover 91 include respective semi-circular grooves 95 and 97, whoseradii are substantially equal to the radius of catheter 20.

Clamp assembly 89 preferably includes a heating element 99 and at leastone temperature sensor 101, which are used to heat distal end 22 ofcatheter 20 to a temperature substantially equal to the temperature ofthe body into which the catheter is to be inserted, and to maintain thedistal end at that temperature during calibration. As is known in theart, the response of coils 60, 62 and 64 to magnetic fields may changeas a function of temperature. For example, when the coils are woundaround ferrite cores, their inductance may change with temperature,which change can introduce errors into the calibration of device 28.Therefore, distal end 22 is typically heated to and maintained at atemperature of 37° C. during calibration, although other temperaturesmay be chosen, for example when catheter 20 is to be used underconditions of hypothermia, such as are generally induced duringopen-heart surgery.

To use jig 77 in calibrating catheter 20, the catheter is inserted ingroove 95, and rotated about its long axis to a desired rotationalorientation, wherein preferably the X, Y and Z catheter axes shown inFIG. 2 are substantially aligned with the magnetic field directionsdefined by radiator coil pairs 83, 79 and 81, respectively. The desiredrotational orientation may be indicated, for example, by fiducial marksor other features (not shown in the figures) on the catheter's outersurface. Alternatively, in preferred embodiments of the presentinvention in which catheter 20 is rotationally symmetrical about itslong axis, the rotational orientation is unimportant, and there is noneed to align the X and Y axes.

After catheter 20 has been inserted and aligned, as necessary, in groove95, cover 91 is then lowered to hold the catheter in place. In thismanner the catheter is fixed in a known orientation relative to themagnetic fields generated by radiator coils 81, 83 and 85.

The respective gains and angular orientations of catheter coils 60, 62and 64 are then calibrated by sequentially activating radiator coilpairs 79, 81 and 83 to generate predetermined, known magnetic fields,and measuring the amplitudes of the signals generated by the cathetercoils.

First, to calibrate the gains of the coils, total amplitudes of therespective catheter coil signals are derived by summing the squares ofthe amplitudes of the signals generated by each of catheter coils 60, 62and 64 in response to each of the coil pairs in turn. Since the magneticfields in the vicinity of coils 60, 62 and 64 have equal andsubstantially uniform components along each of the coil axes 66, 68 and70, the total signal amplitudes will be independent of the respectiveorientations and positions of coils 60, 62 and 64, and will depend onlyon the respective coil gains. Thus, the measured total signal amplitudesmay be used to determine respective normalization factors for coils 60,62 and 64, by dividing the measured amplitudes by expected standardvalues. Subsequently the amplitudes of signals received from these coilsmay be multiplied by the respective normalization factors in order tocorrect for gain variations.

Jig 77 is further used to calibrate the respective angular orientationsof coils 60, 62 and 64 relative to catheter 20, so as to correct fordeviations from orthogonality. The normalized amplitude of the signalgenerated by each of coils 60, 62 and 64 in response to each of themagnetic fields will be proportional to the cosine of the angle betweenthe respective coil axis 66, 68 or 70, and the direction of the appliedmagnetic field. Three such angle cosines, corresponding to thedirections of the tree orthogonal magnetic fields applied by radiatorcoil pairs 79, 81 and 83, may thus be derived for each of catheter coils60, 62 and 64. Since as noted above, catheter 20 is held in clampassembly 87 is such a manner that the X, Y and Z catheter axes aresubstantially aligned with the three orthogonal magnetic fielddirections, the orientations of the coils relative to the catheter axesmay thus be determined.

In preferred embodiments of the present invention, when the Z-axismagnetic field is activated, corresponding in this case to radiator coilpair 83, a normalized amplitude of the signal received from coil 60,S₆₀(Z), is received and measured. The X- and Y-axis fields are similarlyactivated, and corresponding normalized signals S₆₀(X) and S₆₀(Y) arereceived. S₆₀(X), S₆₀(Y) and S₆₀(Z) are used to calculate coil anglecalibration factors for coil 60, which are thereafter recorded incatheter 20 and used in determining the catheter's position andorientation. A similar procedure is used to calibrate coils 62 and 64.

Although the magnetic fields generated by coil pairs 79, 81 and 83 aresubstantially orthogonal and of equal amplitudes, imprecise winding ofthe coil pairs may cause small deviations from orthogonality andequality. These deviations, if not corrected for, may cause errors inthe calibration of catheter 20. Therefore, in preferred embodiments ofthe present invention, a master coil (not shown in the figures) is usedto calibrate jig 77. Preferably this master coil is wound precisely,with a known, predetermined geometrical configuration and dimensions, sothat its sensitivity to an applied magnetic field may be accuratelycalculated, using theoretical methods known in the art.

The master coil is placed in the center of jig 77 in a known,predetermined position and orientation, wherein the axis of the mastercoil is substantially parallel to the direction of the magnetic fieldgenerated by coil pair 79. This coil pair is activated, thereby causingthe master coil to generate an electrical signal. This signal isrecorded and compared with a standard signal value, in order todetermine a calibration factor for coil pair 79. This procedure isrepeated for coil pairs 81 and 83.

When catheter 20 is placed in jig 77 for calibration of the catheter,the signals received from coils 60, 62 and 64 are first corrected toaccount for the calibration factors of coil pairs 79, 81 and 83, beforethe gain normalization and angle calibration factors of the catheter aredetermined.

It will be appreciated that a single master coil may be used tocalibrate multiple jigs, so that all of the calibrated jigs will givesubstantially identical results in calibrating catheters. Furthermore,the same master coil may also be used to calibrate radiator coils, whichproduce magnetic fields for detection of the position of catheter 20inside the body of a subject, in accordance with the 01103 PCT patentapplication.

It will further be understood that a first master coil may be used toproduce and calibrate additional master coils, to be used in turn tocalibrate other jigs and radiator coils. After a jig is calibrated usingthe first master coil, a second master coil is similarly placed in thejig. Signals generated by the second master coil are measured, using theprocedure described above for calibrating the jig. Differences betweenthe signals generated by the second master coil, and those that weregenerated by the first master coil under the same conditions, are usedto determine calibration factors for the second master coil. Thesecalibration factors may be stored in the second master coil, usingdevices and methods similar to those used to store calibration factorsin catheter 20, in accordance with preferred embodiments of the presentinvention.

FIG. 4 shows a preferred embodiment of a jig 80 useful in calibratingthe displacements of coils 60, 62 and 64 relative to catheter tip 26.Jig 80 comprises one or more receptacles 82 into which catheter 20 maybe inserted. Each of receptacles 82 has a known, predetermined depth andangular orientation relative to jig 80. When the catheter is fullyinserted into a receptacle, distal tip 26 of the catheter abuts theinner end of the receptacle. Jig 80 and receptacles 82 are soconstructed that the catheter fits snugly into the receptacles, so thatwhen the catheter is fully inserted, the location and angularorientation of its distal tip are precisely determined with respect to aframe of reference defined by the jig. Preferably, jig 80 also includesa heating element and one or more temperature sensors (not shown in FIG.4), as shown in FIG. 3C and described in reference thereto.

Preferred embodiments of jig 80 further comprise one or more radiatorcoils 84, which generate known, spatially varying magnetic fields, inthe vicinity of device 28. These magnetic fields cause coils 60, 62 and64 in device 28 to generate signals, which are conveyed through catheter20 to signal processing circuits 40, and from these circuits to computer36, as shown in FIG. 1. The computer measures the amplitudes of therespective signals generated by coils 60, 62 and 64, and then determinescorrected values of the amplitudes using gain normalization and coilangle calibration factors, which have preferably been determined asdescribed above. The corrected amplitudes are compared to expectedstandard values, based on the known magnetic field strength at theexpected respective locations of the coils. Deviations between thecorrected, measured amplitudes and the expected standard values are usedto compute displacement correction factors, corresponding to deviationsof the displacements L, d_(y) and d_(z), as shown in FIG. 2, from theirrespective expected values.

Calibration data regarding catheter 20 may be calculated in accordancewith various methods known in the art. For example, in a preferredembodiment of the present invention, the gain normalization, anglecalibration and displacement correction factors are storedelectronically in the form of a look-up table, which is used by computer36 to compute the position and orientation of the catheter's distal tip26.

In an alternative preferred embodiment of the present invention of jig80, the jig includes a plurality of receptacles, each in a different,predetermined position and orientation with respect to the frame ofreference defined by the jig. Radiator coils 84 generate magnetic fieldsthat are substantially identical to those generated by radiator coils(not shown in the figures) that are used to generate external magneticfields for determining the position and orientation of catheter 20inside the body of a subject. Moreover, radiator coils 84 are placed onjig 80 in relative positions and orientations that are substantiallyidentical to the relative positions and orientations of the radiatorcoils that are used to generate external magnetic fields for determiningthe position and orientation of catheter 20 inside the body of asubject.

Catheter 20 is inserted into each of receptacles 82 in turn, andmagnetic fields generated by radiators 84 cause coils 60, 62 and 64 indevice 28 to generate signals, which are conveyed to signal processingcircuits 40 and computer 36. The computer uses these signals to computeposition data, in accordance with methods described in the 01103 PCTpatent application, after first applying gain normalization and coilangle calibration factors, which have preferably been determined asdescribed above. The computed position and orientation of device 28 arecompared to the known, predetermined position and orientation of tip 26in receptacle 82. The differences between the computed and known valuesof position and orientation are used to calculate an empiricaldisplacement correction vector D, and an angle correction vector Θ. Thevalues of D and Θ that are calculated for the plurality of positions andorientations defined by the plurality of receptacles 82 are used togenerated a map of D and Θ as a function of measured position andorientation over the range of positions and orientations defined by jig80. When catheter 20 is subsequently used inside a human body, computer36 applies these correction vectors to the position and orientationsignals generated by device 28, in order to determine the actual,correct position of tip 26.

Calibration vector functions D and Θ may be calculated and recorded inaccordance with various methods known in the art. For example, inpreferred embodiments of the present invention, polynomial functions ofposition coordinates x, y, z and angle coordinates θ₁, θ₂, θ₃ are fit tothe maps of D and Θ by methods known in the art, such as least-squaresfitting. The polynomial coefficients thus derived are storedelectronically and then applied by the computer in determining thecorrection vectors. Alternatively, the values of vector functionsthemselves are stored electronically in the form of look-up tables,which are used by computer 36 to compute the position and orientation ofthe catheter's distal tip 26.

In some preferred embodiments of the present invention, catheter 20 iselectrically isolated from console 34 by isolation circuitry, forexample by one or more isolation transformers in handle 30, as describedearlier in reference to FIG. 1. Such inductive elements and otherisolation circuitry typically introduce non-linearities in signalsconveyed thereby, which may lead to distortion of the signals,particularly analog signals, conveyed to circuitry 40. Thesenon-linearities are preferably measured at the time of cathetercalibration, and the calibration information recorded in catheter 20preferably then includes data relating to signal non-linearitiesintroduced by isolation circuitry.

In preferred embodiments of the present invention, the calibrationcorrection function that is determined in accordance with the methodsdescribed above or using other methods known in the art, is thereafterstored electronically in a memory device, which device is preferably incatheter 20. When the catheter is coupled to console 34, this memorydevice is accessible to the computer in the console.

In one such preferred embodiment of the present invention, illustratedschematically in FIG. 5, connector 44 includes a digital microcircuit 90in which calibration correction function data for catheter 20 iselectronically stored. Microcircuit 90 preferably includes an EEPROM orFlash ROM, but may alternatively include EPROM, PROM, non-volatile RAM,or other types of programmable memory devices known in the art. When acatheter 20 is calibrated, its specific correction data are stored inthe microcircuit located in its console connector 44, which isconveniently accessible to the computer, as will be described below.

In the preferred embodiment shown in FIG. 5, connector 44 furtherincludes pins 92, 94, 96 and 98, which mate with corresponding socketsin receptacle 46. Functional pins 94 couple analog electrophysiologicalsignals conveyed over functional wires 76 to signal processing circuits40. Coil pins 92 couple analog position and orientation signals conveyedby coil wires 72 from coils 60, 62 and 64 to signal processing circuits40 and computer 36, which computes the position and orientation ofcatheter 20. The computer further reads the digital calibrationcorrection function data stored in microcircuit 90 via memory pins 96,and uses these data to in computing the correct catheter position andorientation.

One or more write-enable pins 104 are likewise coupled to microcircuit90. These pins are used to enable programming of the microcircuit withthe desired calibration data. At the time of calibration, thewrite-enable input is enabled, and calibration data are recorded in themicrocircuit. Thereafter the write-enable input is disabled, for exampleby removing the write-enable pin or by connecting it to electricalground, as shown in FIG. 5, so that further calibration data may not berecorded in the microcircuit, and the microcircuit functions in aread-only mode.

Alternatively, in preferred embodiments of the present invention whereinmicrocircuit 90 comprises an EEPROM device, the write-enable input maybe disabled by sending a write-protect command to the device. Thiscommand may be reversible or irreversible.

In other preferred embodiments of the present invention, microcircuit 90comprises a device incorporating password-secured access control, andwrite-access to the microcircuit requires that an appropriate passwordfirst be entered. For example, in one such preferred embodiment,microcircuit 90 comprises a Password Access Security Supervisor (PASS™)X76F041 SecureFlash ROM device, manufactured by Xicor, Inc. Themicrocircuit is programmed with calibration data at the time ofmanufacture, and thereafter operates in a “read access only” mode, withall write operations locked out, or in a “read access and program only”mode, in which certain data, but not calibration data, may be written tothe device, as will be described below. Changing the mode of operationof the microcircuit requires that an appropriate password be entered,which password is generally unavailable to users of the system.

In another preferred embodiment of the present invention, microcircuit90 comprises an EPROM or PROM device, which is contained in the catheterconnector, and the input and output connections of the EPROM or PROM arecoupled to pins of the connector. Calibration data are recorded in theEPROM or PROM at the time of manufacture using a suitable programmingdevice, not shown in the figures, which receives data from the computerused in calibration. The programming device is connected to catheterconnector 44 and programs the EPROM or PROM by inputting digital signalsthereto through the connector. Thereafter, the EPROM or PROM may not bere-programmed.

In some preferred embodiments of the present invention, data recorded inmicrocircuit 90 include a calibration code, which is encrypted inaccordance with methods known in the art, so as to ensure that thecalibration data have not been altered or corrupted. Preferably thecalibration code includes a checksum. When the user connects catheter 20to console 34, computer 36 reads the calibration code and compares thecode with pre-programmed values. If the code does not match the desiredpre-programmed value, the computer causes a message to be displayed bydisplay 42 indicating that the catheter may not be appropriatelycalibrated. The computer may further cause the system to cease operationuntil a catheter having a code matching the desired pre-programmed valueis connected thereto.

Preferably the calibration code is encrypted using a method thatprevents decryption by unauthorized parties, for example the RSAencryption scheme, using a public key and a private key, or othermethods known in the art. When a method such as RSA encryption is used,the private key is known only to authorized manufacturers of thecatheter, so as to prevent the possible use of unauthorized substitutesof possibly inferior quality.

In further preferred embodiments of the present invention, data recordedin microcircuit 90 include an expiration date and time, after which thecatheter may not be used. When a user connects catheter 20 to a console34, computer 36 reads the expiration date and time and compares then tothe actual date and time, generated, for example, by a real-time clockcircuit. If the expiration date and time have passed, the computercauses a message to be displayed by display 42 indicating that thecatheter is unsuitable for further use. The computer may prevent furtheroperation until a catheter having a valid expiration date and time isconnected thereto.

Preferably the expiration date and time arm recorded by computer 36 byprogramming microcircuit 90 in catheter 20 when the catheter is firstused. Thus, when catheter 20 is connected to console 34 for the firsttime, computer 36 detects that no expiration date and time have yet beenrecorded in microcircuit 90, and programs the microcircuit with theappropriate expiration data and time, at a pre-set interval after theactual date and time. The pre-set interval is preferably determined bythe manufacturer, based on the expected useful life of the catheter.

In preferred embodiments of the present invention in which microcircuit90 comprises a device including access control circuitry, such as theaforementioned X76F041 device, the microcircuit is programmed so that amemory location therein is operable in a “read access and program only”mode. The mode may be changed only by entry of an appropriate password,which is generally not available to users of the system. In the “readaccess and program only” mode, a number stored in the memory locationmay be decreased, by changing a bit from “1” to “0”, but not increased,since the microcircuit as programmed will not permit a “0” to be changedto a “1”. Preferably the memory location is set at the time ofmanufacture to contain a maximum value, i.e., all bits set to “1”. Then,as described above, at the time catheter 20 is first used, computer 36programs the microcircuit with the appropriate expiration time and dateby changing one or more bits in the register from “1” to “0”.Thereafter, the expiration date cannot be changed to any later date(unless the correct password is first entered).

Alternatively or additionally, microcircuit 90 comprising access controlcircuitry, as described above, may be used to track the number of timescatheter 20 has been used, in a manner that is protected from possibletampering or error by a user thereof. Preferably, a record correspondingto the number of times catheter 20 may be used is stored in a memorylocation in the device at the time of manufacture, and the microcircuitis programmed so that this memory location is operable in the “readaccess and program only” mode, as described above. Each time thecatheter is used, computer 36 reads the record in the memory locationand reduces it by changing one or more bits therein from “1” to “0”.When all the bits in the record are equal to zero, or the record reachessome other predetermined minimum value, the computer causes a message tobe displayed to the user indicating that the catheter is unsuitable forfurther use and, preferably, prevents further operation until a suitablecatheter is connected thereto.

Similarly, either alternatively or additionally, microcircuit 90 may beused to track the duration of use of catheter 20. In this case, a recordcorresponding to the duration of use of the catheter is stored in a“read access and program only” memory location in the microcircuit.While the catheter is in use, at regular, predetermined intervals,computer 36 reads the record and reduces it by changing one or more bitstherein from “1” to “0”. When the entire record reaches zero, or someother minimum value, further operation is prevented, as described above.As noted earlier, the low-level analog signals conveyed from coils 60,62 and 64 over coil wires 72 must generally be protected frominterference due to other analog signals in functional wires 76 anddigital signals conveyed to an from microcircuit 90. Therefore, inpreferred embodiments of the present invention, as shown in FIG. 5,connector 44 includes electromagnetic shields 74, which are coupled toground via pin 98 on the connector.

In another preferred embodiment of the present invention, shields 74 areactive shields, which are driven by noise canceling circuitry (notshown).

It will further be appreciated that by locating microcircuit 90 inconnector 44, the length of electrical conductors carrying digitalsignals in proximity to the low-level analog signals in coil wires 72 isheld to a minimum, thereby reducing the possibility of electricalinterference with the low-level signals.

In a preferred embodiment of the invention, catheter 20 is a wirelesscatheter which is not physically connected to the signal processingand/or computing apparatus. Rather, a transmitter/receiver is attachedto a proximal end of the catheter and all electronic signals generatedby the catheter are transmitted by the transmitter/receiver. Thetransmitter/receiver communicates with the signal processing and/orcomputer apparatus using wireless communication methods, such as IR(infra red), RF or acoustic transmissions. One benefit of this type ofconfiguration is that the catheter, which is inserted into the(electrically sensitive) heart can easily be made electrically floatingand/or completely isolated from any external (to the body) electricalpower source. Another benefit is a reduction in the amount of cablingand wiring with which one of the many operators might get entangledand/or accidentally pull out of the body. Still another advantage is theease of sterilizing and maintaining the sterility of such a catheter,since the entire catheter may be sterilized as a single unit. The powersupply for such a catheter is preferably permanently enclosed within thecatheter. When the catheter is used, the power supply is activated andit is capable of powering the catheter for a limited amount of time.Alternatively, the power supply is a rechargeable power supply which maybe recharged after each use, thereby allowing multiple uses of the samecatheter.

In a preferred embodiment of the invention, the proximal end of thecatheter, which includes the transmitter/receiver, is attached to anoperator's belt. Preferably, there is a handle disposed on the catheter,a few feet away from the proximal end thereof, for control of thecatheter. As can be appreciated, when such a catheter is used forablation or for infusion of materials into the body, it is preferablymomentarily connected to an external device, such as an RF generator.

Although the above preferred embodiments have been described withreference to calibration of position and orientation sensing apparatus,in other preferred embodiments of the present invention, calibrationdata stored in catheter 20, and specifically in microcircuit 90, mayrelate to other aspects of the catheter. For example, in some preferredembodiments of the present invention, calibration data relating to aphysiological sensor, actuator or therapeutic tool are stored in thecatheter. In another preferred embodiment of the present invention,calibration data may be stored in the catheter regarding the gain of apiezoelectric motion control device used in steering the catheter'sdistal end.

It will be appreciated that the preferred embodiments of the inventiondescribed above are cited by way of example, and the full scope of theinvention is limited only by the claims which follow.

What is claimed is:
 1. A method of calibrating a probe for insertioninto the body of a subject, comprising: providing a probe having aprogrammable microcircuit and a distal end, the distal end including afunctional portion adjacent to a distal tip of the distal end forperforming diagnostic and/or therapeutic functions, the distal end alsoincluding a position signal generating device for generating signalsused to determine the position of the probe within the subject's body;determining calibration data for the position signal generating devicewith respect to the distal tip of the probe; and programming themicrocircuit so as to record the calibration data in the microcircuit.2. A method in accordance with claim 1, and comprising encrypting acalibration code and programming the microcircuit therewith.
 3. A methodin accordance with claim 2 comprising: reading the encrypted code; andnotifying a user of the probe if the encrypted code does not match apredetermined code.
 4. A method in accordance with claim 3 comprising:reading the encrypted code; and ceasing operating of the probe if theencrypted code does not match a predetermined code.
 5. A method inaccordance with claim 1, wherein programming the microcircuit includessetting a usage record.
 6. A method in accordance with claim 5, whereinthe usage record is indicative of a permitted use date of the probe. 7.A method in accordance with claim 5, wherein the usage record isindicative of how many times the probe may be re-used.
 8. A method inaccordance with claim 5, wherein the usage record is indicative of aduration of time during which the probe may be operated.
 9. A method inaccordance with claim 5, wherein programming the microcircuit includesrestricting access to the usage record, so that availability of theprobe to a user thereof may be reduced, but not increased.
 10. A methodin accordance with claim 9, wherein restricting access to the usagerecord comprises allowing one or more bits in the record to be changedfrom a first value to a second value thereof, but not from the secondvalue to the first value.
 11. A method in accordance with claim 1,wherein restricting access to the usage record comprises setting apassword.
 12. A method in accordance with claim 1, wherein thecalibration data relate to the position signal generating device, whichgenerates signals responsive to the position or orientation of theprobe.
 13. A method in accordance with claim 12, wherein the signalgenerating device has a gain, and the calibration data include datarelating to the gain of the device.
 14. A method in accordance withclaim 12, wherein the calibration data include data relating to anangular orientation of the signal generating device.
 15. A method inaccordance with claim 12, wherein the calibration data include daterelating to a positional displacement of the signal generating device,relative to the probe.
 16. A method for programming a probe forinsertion into the body of a subject, the method comprising the stepsof: providing a probe having a programmable microcircuit and a distalend, the distal end including a functional portion adjacent to a distaltip of the distal end for performing diagnostic and/or therapeuticfunctions, the distal end also including a position signal generatingdevice for generating signals used to determine the position of theprobe within the subject's body; and programming the microcircuit withan expiration date and time.
 17. The method according to claim 16,including determining a pre-set interval for the expiration date andtime based on the actual date and time when the probe is first used. 18.A method for programming and tracking usage of a probe for insertioninto the body of a subject, the method comprising the steps of:providing a probe having a programmable microcircuit and a distal end,the distal end including a functional portion adjacent to a distal tipof the distal end for performing diagnostic and/or therapeuticfunctions, the distal end also including a position signal generatingdevice for generating signals used to determine the position of theprobe within the subject's body; programming the microcircuit with anumber of times that the probe may be used; and tracking with themicrocircuit the number of times that the probe is used.
 19. The methodaccording to claim 18, further comprising wherein the number of timesdefines a minimum value and preventing further operation of the probeupon reaching the minimum value.
 20. The method according to claim 19,further comprising displaying a message to the user that the probe isunsuitable for further use upon reaching the minimum value.
 21. A methodfor programming and tracking the usage of a probe for insertion into thebody of a subject, the method comprising the steps of: providing a probehaving a programmable microcircuit and a distal end, the distal endincluding a functional portion adjacent to a distal tip of the distalend for performing diagnostic and/or therapeutic functions, the distalend also including a position signal generating device for generatingsignals used to determine the position of the probe within the subject'sbody; storing a record in the microcircuit corresponding to a durationof use for the probe, the record including a minimum value; reducing therecord for each use of the probe; and preventing further operation ofthe probe when the record reaches the minimum value.